Search

Stable isotope and geochemical data are used here to differentiate between contemporaneous abiotic and microbial processes leading to formation of modern carbonate- (calcite, aragonite and magnesite) and silicate-rich (kerolite) mineralization in basaltic sea caves on the island of Kauai, Hawaii. Strontium isotope and Ca/Sr ratios in meteoric water and cave carbonates suggest that the majority of Sr and Ca are derived from rock–water interaction within the host basalts situated above the caves. Oxygen and hydrogen isotope ratios and chemical compositions of cave and surface waters indicate that evaporation does not control cave-water composition. However, evaporation of drops and thin films of water in microenvironments can lead to precipitation of some phases. This behaviour is suggested by the covariance in δ18O and δ13C values of some carbonates, especially magnesite, which is considered to be a late-stage evaporative precipitate. Modelling of water evolution suggests that evaporation can be a cause of supersaturation for magnesite, kerolite and some Ca carbonates. However, the highly elevated δ13C values (up to +8.2) of some Ca carbonates, compared to average dissolved inorganic carbon δ13C values (~−12), are best explained as the product of microbial photosynthesis, in particular by cyanobacteria, present in the upper layers of active microbial mats on cave surfaces. The preferential uptake of 12C by cyanobacteria is recorded in the low δ13C values (−29.1 to −22.6) of organic matter in mats and mineralized microbialites. The resulting 13C-enrichment of dissolved inorganic carbon is recorded in the elevated δ13C values of these Ca carbonates. A positive correlation exists between the δ13C values of the carbonates and coexisting organic matter. The large enrichment in 13C of carbonate minerals, relative to dissolved inorganic carbon, and its covariance with the δ13C values of coexisting organic matter are useful for identification of carbonate-rich mineralization resulting from autotrophic microbial activity.

The occurrence of kerolite in association with various secondary Ca-Mg carbonate mineral deposits (speleothems) was identified in basaltic sea caves on the island of Kauai, Hawaii. Kerolite is the dominant clay mineral in the deposits. X-ray diffraction (XRD) peaks of the kerolite are characteristically broadened indicating its extremely poor crystallinity. Few changes were observed in the XRD patterns of this kerolite when it was subjected to various humidity, temperature and ethylene-glycol treatments. The crystals appear as flaky masses with irregular or jagged edges in scanning (SEM) and transmission electron microscopy (TEM). Electron probe and energy dispersive X-ray (EDX) microanalysis show that the clay material is dominated by Mg-Si-O, with minor amounts of Al and Ca in some samples. The chemical composition, thermal analysis and TEM observations suggest that smaller amounts of an amorphous serpentine-like phase are mixed with the kerolite. Kerolite is often the only mineral associated with poorly mineralized, actively-growing microbial mats in these caves and it is common in completely lithified microbial mats. The latter commonly have microstromatolitic structures with kerolite as a dominant phase. These features suggest that kerolite formation is at least in part a result of microbial activity. The abundant extracellular polymers of the mat-forming bacteria bind and concentrate ions (Mg2+, silica) from solution and serve as nucleation sites for kerolite precipitation. Conditions within the mats also probably lead to formation of Mg-Si-gels, amorphous Mg-silicate precursors and ultimately kerolite. Evaporation of the cave solutions may also contribute to kerolite formation.

Aragonite, calcite, monohydrocalcite, dolomite, magnesite, hydromagnesite, and talc‐like kerolite have been identified in speleothems from basaltic caves on the island of Kauai, Hawaii. These secondary minerals have crystallized in the presence of freshwater seeping from the basaltic host rock, and the mineralization process is ongoing at some locations. The deposits are extremely variable in macroscopic character, ranging from millimeter‐thin, powdery coatings to hard crusts several centimeters thick. Many deposits are associated with actively forming microbial mats, and stromatolitic structures are common in the thicker deposits. Formation of the mineral deposits is a result of a combination of direct precipitation from solution by evaporation and CO2 degassing, alteration of precursor minerals, and (or) microbial processes and physicochemical conditions within the mats. These unusual deposits provide important insights into the mineralogy and geochemistry of low‐temperature alteration and carb...

Modern microbial mats and microbialites are described from basaltic sea caves on the island of Kauai, HI. The mats grow on the ceilings and walls in the photic zone of several open caves where fresh water seeps out of the rock. Scanning (SEM) and transmission electron microscopy (TEM) showed that the active mats are dominated by filamentous and nonfilamentous cyanobacteria in the surface layers and heterotrophic bacteria in deeper layers. Energy dispersive X-ray analysis revealed that copious amounts of extracellular polymeric substances (EPS) are rich in Mg, Si, O, and Ca, likely concentrated from solution. Petrographic microscopy and electron microprobe analysis of the mineralized microbialites showed textures reminiscent of stromatolitic laminations, consisting mainly of alternating calcium carbonate (calcite and aragonite) and magnesium-rich silicate (kerolite). Thin coatings rich in magnesite, hydromagnesite and monohydrocalcite surround the microbialites on the rock surfaces and are likely inorganic in origin. Within the mats, minerals tend to form and concentrate within, or around, dense matrices of EPS. Microenvironments with geochemical conditions favorable for mineral crystallization likely develop in the mats as a result of the mucilaginous extracellular material and the development of bacterial microcolonies. In addition, copious amounts of extracellular polymers bind ions from solution and provide nucleation sites for mineral crystallization and growth. This combination of biological and inorganic processes can explain the occurrence of the secondary minerals in these caves, as well as the stromatolitic textures of the microbialites.